1. Field of the Invention
The present invention relates generally to apparatus for receiving and using the energy present in solar radiation. The invention relates more specifically to apparatus used to preheat a working fluid for subsequent solar reaction, and to apparatus used for efficient solar absorption in the absence of driven unforced fluid flow.
2. Description of the Related Art
It is well known to utilize blackbody devices for the absorption of solar energy. In order to create a solar absorption device that is economically feasible, many different collector arrangements and working fluids have been proposed. For example, U.S. Pat. No. 3,987,781 to Nozik discloses the use of a blackbody receiver surrounded by a transparent shell that is coated with cadmium stannate. The object of the invention is to transmit solar radiation and reradiate infrared radiation back into the receiver.
By virtue of this construction, the invention disclosed by Nozik allows the blackbody to absorb the solar radiation through the transparent shell, while somewhat alleviating the problem of reradiation from the blackbody to the surrounding atmosphere. It is important to note that this invention, and other prior art devices, provide for energy absorption in but a single stage.
Even with the use of a radiation-selective surface, reradiation is a problem at high working temperatures with conventional solar absorption receivers. Such is the case because the blackbody must become very hot in order to heat the working fluid to the desired temperature. The blackbody will then radiate large amounts of energy to any surface that is at a lower temperature than itself. The energy transferred, or lost, through reradiation is proportional to the temperature difference between the blackbody and any surrounding surfaces. Thus, the higher the blackbody temperature, the greater the losses that may be attributed to reradiation.
Although conventional blackbody and other direct absorption receivers are satisfactory in certain respects, there are inherent limitations on the use of surface absorption of solar radiation to drive external heat engines. Specifically, the receiver cavity in such systems generally operates at a higher temperature than the working fluid driving the external heat engine, since a gradient is required to achieve the heat transfer from absorbing surface to working fluid. The efficiency of a blackbody or direct absorption receiver decreases with increasing temperatures due to reradiation in the infrared. Consequently, the increase in efficiency of the associated heat engine achieved at higher operating temperatures is ultimately offset or lost entirely in the system combination.
In addition, difficulties with absorbing-surface thermal stress can arise in the presence of high temperatures and high light fluxes in conventional blackbody or direct absorption receivers, providing a serious limitation on the capability of such devices at elevated power levels.
Inventions that have as their objects the generation of electrical power through the absorption of solar radiation and subsequent dissociation of the halogen constituents of a working fluid include U.S. Pat. Nos. 4,848,087 and 4,945,731.
U.S. Pat. No. 4,848,087 entitled "Solar Augmented Power System" discloses an energy conversion system in which a focused beam of radiation is employed to induce a reactive substance to produce reaction products at elevated temperatures and pressures. The pressurized materials are then controllably exhausted and introduced into a means for converting the heat and pressure of the pressurized materials into other useful work.
U.S. Pat. No. 4,945,731 entitled "Absorbing Fluid Receiver for Solar Dynamic Power Generation and Solar Dynamic Power System" discloses a receiver for a solar dynamic power system. The receiver has a hollow, cylindrical containment with a window for admitting solar radiation in the receiving space to heat a working fluid. The working fluid comprises a radiant energy absorber selected from halogens and interhalogens. Means are provided for coupling the working fluid with a heat engine for the purpose of generating electrical power.